The metal containers which are used for beverages and foods are roughly divided into two-piece cans and three-piece cans.
In the process of production of two-piece cans such as DI cans, the can is drawn and ironed, then the inside of the can is coated and the outside of the can is coated and printed on.
In the process of production of three-piece cans, the surface corresponding to the inside surface of the can is coated, the surface corresponding to the outside of the can is printed, then the can body is welded.
In each type of can, a coating process is essential before and after the canmaking. For the coating, a solvent-based or water-based coating is used, then the coating is baked on.
In the coating process, the waste solvent derived from the coating is discharged as industrial waste. Exhaust gas (mainly carbon dioxide gas) is released into the air. In recent years, efforts have been made to reduce the industrial waste and exhaust gas for the purpose of protecting the global environment.
Among these efforts, the technique of laminating a film to take the place of coating has come into attention and has been rapidly spreading.
In two-piece cans, numerous methods of production of cans which laminate films to make the cans and inventions relating to these have been devised (for example, PLT's 1 to 4).
As inventions relating to three-piece cans, for example, PLT's 5 to 8 may be mentioned.
On the steel sheet used as the base material for the laminate film, in most cases, a chromate film obtained by electrolytic chromate treatment is being used. A chromate film has a two-layer structure of a metal Cr layer over which a hydrated Cr layer is formed.
The laminate film (if a film with a binder, the binder layer) secures the adhesion with the steel sheet and the wettability with the coating through the hydrated Cr layer of the chromate film. The mechanism of manifestation of this adhesion has not been clarified in detail, but is said to be a result of the hydrogen bonds of the hydroxyl groups of the hydrated Cr and the carbonyl groups of the laminate film or ester groups or other functional groups.